An electrophotographic method is usually a method in which an electrostatic latent image is formed on an electrostatic latent image-carrier comprising a photoconductive light-sensitive material by charging and imagewise exposure to light, and then developed with a toner composition which contains a colorant dispersed in a binder resin, and the toner image thus obtained is transferred to a support such as a transfer paper and fixed.
As such toner compositions to obtain visible images, those comprising a black colorant such as carbon black dispersed in a binder are generally used. In recent years, color toners comprising a binder resin and a blue pigment, a magenta pigment or a yellow pigment dispersed in the binder resin have been used. Among these color toners are those for obtaining polychromatic images by the electrophotographic method. Usually, blue, magenta and yellow toners are used.
In the case of such color toners, it is also required that a color image as transferred to and fixed on a sheet for an overhead projector (hereinafter referred to as "OHP") can be clearly and sharply projected on a screen.
A color toner composition generally comprises a binder resin and a colorant as main components, and various additives. Binder resins generally used are polystyrene, a styrene-(meth)acrylic acid ester copolymer, a styrene-butadine copolymer, polyester, an epoxy resin, a cumarone indene resin, and the like. In particular, a polyester resin has been began to use because it is relatively good in fixability and is good in transmittability of OHP.
A polyester resin, however, is inherently of high negative chargability and, therefore, in the case of a toner containing the polyester resin, the amount of electricity charged is increased and becomes excessive during the period of use thereof after mixing with a carrier in a developing machine. If the amount of electricity charged is increased excessively, the binding force between toner particles and carrier particles becomes strong and the toners are not used in development of an electrostatic latent image on the light-sensitive material. In some cases, they are developed in combination with carrier particles. If they are transferred to paper, for example, problems arise in that the density of transferred image is decreased and white dots are formed.